In order to cleanse new healing stones, can I set them on my dash in my car in order for them to get 3 hours of direct sun, or should I set them outside in direct sun?

Idk. Do what ever makes you happy...

Sunlight, moonlight, rain water all are good as long as the stones can handle those sources.. Some stones you can get wet.. I like to sage them really good and set them in the light of the full moon. Works like a charm ;-)

They may get to hot in the car and they can crack. Put them outside in the morning and take in before noon, before it gets to hot. I cleanse mine by holding in my hand under cold running water for a couple of minutes. But there are a couple of stones you can't do that with.

It depends on the stones. The sun can do damage to some of them. You should look up each stone and see what it says about it.